Symptom: Post-flight, engine still hot, moved the prop and heard a faint scraping noise. The noise was much diminished, almost non-existent when the engine was cold. Put another way, it wasn't notable if/when the prop was moved during a preflight.
Here's the interesting part. Removed cowl, removed the belt from the alternator pulley, spun the alternator by hand without any noise. Pushing and pulling the shaft axially made no difference. Otherwise wiggling the pulley detected no abnormality.
Hmmm.
Removed the alternator from the engine. Padded the jaws on my big shop vice with some soft aluminum, and clamped the pulley in the vice so the shaft pointed straight up, brush end on top. Now the problem was evident. With the pulley firmly fixed, the brush end of the alternator case could be moved roughly a millimeter total in any axis.
The problem was an enlarged bearing bore in the aluminum rear cover. The point here is that simply spinning the rotor isn't an inspection which will detect the problem. The bearing itself had not failed, so it rotated smoothly. The lever ratio (distance from the front bearing to the rear bearing/distance from the pulley centerline to the front bearing) is so large that hand-manipulating the pulley won't generate enough force to detect the problem. The weight of the rotor when horizontal also plays a role.
Scraping when hot, but not cold? I assume thermal expansion of the big Lycoming ring gear carrier, which would significantly tighten the belt in operation, apparently providing enough force to pull the rear end of the rotor into fleeting contact with the stator.
There is one other clue worth noting. This alternator had a bit more black dust in the vicinity of the rear cover. I assume it was brush dust. With the rear bearing floating around in an enlarged bearing bore, the brushes would have been subjected to more than normal movement in their holders.
Plane Power knows there is a problem. Their service rep cheerfully admits to it, but says the cause is too much belt tension. Some of us think that's unlikely, but I'm not going there with this thread. Nothing we say will fix the issue; that's up to Hartzell now. Until it is fixed, best if users learn how to safely live with it, i.e. catch during maintenance. And yes, we already ran a poll; B&C units appear to be far more reliable. In fairness, this tip would apply to any alternator, including B&C, and perhaps I'm just the last guy to have learned it the hard way.
Here's the interesting part. Removed cowl, removed the belt from the alternator pulley, spun the alternator by hand without any noise. Pushing and pulling the shaft axially made no difference. Otherwise wiggling the pulley detected no abnormality.
Hmmm.
Removed the alternator from the engine. Padded the jaws on my big shop vice with some soft aluminum, and clamped the pulley in the vice so the shaft pointed straight up, brush end on top. Now the problem was evident. With the pulley firmly fixed, the brush end of the alternator case could be moved roughly a millimeter total in any axis.
The problem was an enlarged bearing bore in the aluminum rear cover. The point here is that simply spinning the rotor isn't an inspection which will detect the problem. The bearing itself had not failed, so it rotated smoothly. The lever ratio (distance from the front bearing to the rear bearing/distance from the pulley centerline to the front bearing) is so large that hand-manipulating the pulley won't generate enough force to detect the problem. The weight of the rotor when horizontal also plays a role.
Scraping when hot, but not cold? I assume thermal expansion of the big Lycoming ring gear carrier, which would significantly tighten the belt in operation, apparently providing enough force to pull the rear end of the rotor into fleeting contact with the stator.
There is one other clue worth noting. This alternator had a bit more black dust in the vicinity of the rear cover. I assume it was brush dust. With the rear bearing floating around in an enlarged bearing bore, the brushes would have been subjected to more than normal movement in their holders.
Plane Power knows there is a problem. Their service rep cheerfully admits to it, but says the cause is too much belt tension. Some of us think that's unlikely, but I'm not going there with this thread. Nothing we say will fix the issue; that's up to Hartzell now. Until it is fixed, best if users learn how to safely live with it, i.e. catch during maintenance. And yes, we already ran a poll; B&C units appear to be far more reliable. In fairness, this tip would apply to any alternator, including B&C, and perhaps I'm just the last guy to have learned it the hard way.
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